1. Field of the Invention
The present invention relates to RF power amplifiers and amplification methods. More particularly, the present invention relates to feed forward amplifiers and methods for controlling feed forward amplifiers.
2. Description of the Prior Art and Related Background Information
RF amplifiers are devices that attempt to replicate a RF signal present at its input, producing an output signal with a much higher power level. The increase in power from the input to output is referred to as the ‘gain’ of the amplifier. When the gain is constant across the dynamic range of the input signal, the amplifier is said to be ‘linear’. Amplifiers have limited capacity in terms of power delivered because of gain and phase variances, particularly saturation at high power, which makes all practical amplifiers nonlinear when the input power level varies. The ratio of the distortion power generated relative to the signal power delivered is a measure of the non-linearity of the amplifier.
In RF communication systems, the maximum allowable non-linearity of the amplifier is specified by government agencies such as the FCC or the ITU. Because amplifiers are inherently nonlinear when operating near saturation, the linearity requirements often become the limitation on rated power delivering capability. In general, when operating near saturation, the linearity of the amplifier degrades rapidly because the incremental signal power delivered by an amplifier is proportionally less than the incremental distortion power generated.
Various compensation approaches are conventionally applied to reduce the distortion at the output of the system, which in turn increases the rated power delivering capability. The preferred approach is feed forward compensation. In feed forward RF power amplifiers an error amplifier is employed to amplify main amplifier distortion components which are then combined out of phase with the main amplifier output to cancel the main amplifier distortion component. In general, feed forward compensation provides the power capability of the main amplifier and the linearity of the error amplifier.
The performance of a feed forward amplifier may typically be analyzed based on two cancellation loops. Loop1, called the carrier cancellation loop, includes the RF input and the main amplifier. In addition to the main amplifier signal output the first loop provides a distortion signal obtained by sampling the main amplifier output and combining it with an out of phase sample of the RF input signal. Conventionally, the gain and phase of the signal in loop1 are controlled to ideally provide a distortion signal with the input RF carrier component completely cancelled and only the distortion component remaining. Loop 2 is typically referred to as the error cancellation loop or auxiliary path loop. In loop 2 the distortion component provided from loop 1 is amplified by the error amplifier and injected back into the main path at an error coupler to cancel the distortion component in the main path and ideally provide a distortion free signal at the output.
In many cases, a pilot signal is injected at an offset frequency from the signal bandwidth inside the main amplifier signal path. This pilot acts as a known level of distortion at a known frequency. Isolation, detection, and cancellation of this pilot signal makes loop 2 gain adjuster and phase adjuster control easier. Traditional feed forward controllers minimize injected pilot power measured at a loop 2 test coupler configured after the error coupler. Pilot minimization is also referred to as pilot cancellation.
A problem with the traditional feed forward approach is the component cost and signal loss in the second loop delay. This delay may take the form of a delay cable or a delay filter. The longer the delay, the higher the delay cost and signal loss. The delay loss reduces the output power capability of the feed forward amplifier system by attenuating the main amplifier output. To compensate for this loss, a larger more costly main amplifier must be used. To reduce signal losses and component cost, the delay can be reduced or removed. Reducing or removing second loop delay introduces delay mismatch with the error path, which includes the error amplifier. This delay mismatch narrows the bandwidth of second loop cancellation, producing full cancellation at only one frequency. This cancellation bandwidth narrowing is caused by the phase shift with frequency error introduced by the delay mismatch. When controlling the second loop by reducing pilot power, as is done in traditional feed forward systems, the center frequency of second loop cancellation will be placed on top of the pilot frequency. Since the pilot is offset in frequency from the signal bandwidth, second loop cancellation will be lower in value and asymmetric to the signal bandwidth. Generally speaking, the distortion characteristics of the main amplifier will produce symmetrical spectral distortion characteristics about the input signal bandwidth, with the largest distortion nearest the signal in frequency. To meet output signal spectrum requirements, the bandwidth of second loop cancellation center frequency should be placed at the center frequency of the input signal.
Therefore, a need presently exists for an improved delay mismatched feed forward amplifier system which can address this problem of frequency dependent second loop cancellation and optimize system performance.